I teach an introductory course on mammal identification and natural history. This blog serves as a place for all of those stories, photos, facts and fun stuff that simply won't fit in the course. Type in your email below to follow this blog!

Follow by Email

Sunday, October 14, 2012

Out on a limb...

Sometimes I am reluctant to pronounce with authority what is happening in my camera trap photos (Going out on a limb so to speak). But this time, I feel I can safely say I have solved "The Mystery of the Barkless Branch". When I came upon the branch in question, my first thought was "This is the work of squirrels!" But I had never seen anything quite like it before. I have seen thinner branches stripped and I have seen smaller patches barked, but never have I seen such a large limb laid so bare. The second thing that gave me pause was this was a downed limb. By the looks of it, it seemed dead rather than a living branch ripped from a tree. So I was puzzled as to who did it and why. A camera trap was in order! Now, someplace on this laptop or in one of the dozen SD cards floating around my workspace are the photos that I took of that limb to show you good people the extent of the damage. I will add them if I ever find them.

Squirrel bite marks
(Stony Brook SP, NY)

As for WHY, I felt there were three options. First, it could be feeding. Squirrels and other rodents (as well as rabbits) are known to "bark" trees for food. The prize is the tender cambium layer under the bark. Take your fingernail and score a young tree and you will easily find the green cambium layer. Second, this could be a marking behavior. As it so happens, I have been increasingly looking for squirrel bites after first learning of them in February. I wrote about woodchuck bites here. (and remember that woodchucks ARE squirrels). Here is what a typical squirrel bite looks like -->
Notice that it is a long stripe on the tree. The purpose of this biting seems to be to transfer scent from glands on the cheek rather than to feed. The bites themselves never make it all the way to the cambium layer. So what I was seeing on the branch in front of me was nothing like this. But I am no squirrel-marking expert so I was reluctant to eliminate any possibility. Finally, I felt it was possible that the bark was being collected as nesting or winter den material. The only flying squirrel nest I have ever seen was in a bluebird box and was completely made of shredded bark.

Look closely to see the individual incisor marks of the squirrels.

One last shot to give you an idea of how large this stripe is. According to Mark Elbroch, the squirrel bites are likely to be on the sheltered side as this one is -- they are taking advantage of the natural lean of the tree to keep the scent from being washed away too quickly.

As for the WHO, I had several suspects in mind. I was pretty confident this wasn't a bird, but I considered woodpeckers. However, the wood was really not compromised at all as one would expect from a woodpecker. That left mammals and more specifically rodents (this just wasn't the work of rabbits). I was thinking gray squirrel since the are bigger and this was some big damage and the woods were primarily deciduous; the prime habitat for gray squirrels. I remember thinking Eastern chipmunk as well and as it turns out we never got a single image of chippies.

So our game of clue was upon us. Was it Professor Pileated with the candlestick in the library? Or Ms, Squirrel in the kitchen with a lead pipe? Time would tell.....

Here are the results:

First, the most images obtained were of mice. I never saw the mice engaged in any behavior that would account for the damage. It appeared they were using the log as a highway. I made a single composite photo of several mice just for fun:

But the second most common critter was the red squirrel. I didn't really consider this prime red squirrel habitat so I didn't really think of him as a suspect, but perhaps that is exactly WHY he was eating bark. In prime habitat, the red squirrel is eating pine seeds. I am under the impression that bark eating in squirrels is a sign that there is not enough of their preferred food. We had a very poor beech nut and acorn crop this year so maybe that had something to do with this behavior as well.

Red Squirrel
(Wayland, NY 10/12)

I have done this once before but here is a composite image of a gray and red squirrel so you can easily compare their sizes.

Red and gray squirrel compared
(Wayland, NY 10/12)

But its the video that really helps tell the story of what is happening: